1. Field of the Invention
The invention relates to charged particle accelerators and, more particularly, to those designed to accelerate protons with a view to obtaining a high-energy beam.
A known way of treating malignant tumors is to subject them to radiation of varying intensity. Hence, sources of X-rays or charged particles such as electrons or protons are used. In the case of charged particles, the protons have the advantage of a better definition of the target volume owing, firstly, to their penetration to a well-defined depth (Bragg's peak) and secondly to the absence of penumbra.
2. Description of the Prior Art
Hence, it is increasingly being proposed to use proton accelerators, but the energy required is very high given the limited penetration of these protons into the body, this penetration being of the order of 10 centimeters per 100 MeV of energy. Hence, the making of a 250 MeV linear proton accelerator of a conventional standing wave type entails lengths that may attain 28 meters. A proton accelerator such as this has been described, for example, in the paper by R. W. Hamm, K. R. Crandall and J. M. Potter, "Preliminary Design of a Dedicated Proton Therapy Linac", presented to the Particle Accelerator Conference, San Francisco, 6th to 9th May 1991.
An object of the present invention, therefore, is to make a proton accelerator for medical or other uses, the dimensions of which are reduced by a factor of two to three and the cost of which is reduced by an even greater factor owing to the simplicity of the structure implemented. It must be noted that applications other than medical ones may benefit from the simplification provided by the present invention at the cost of a particular adaptation, notably as regards the diameter of passage of the proton beam which will be taken to more than ten millimeters instead of the four millimeters required for a medical application.
To achieve this object, the invention implements one or more accelerator structures of the type using a travelling wave with magnetic coupling, either with forward propagation of the accelerator wave or with backward propagation of said wave with respect to the direction of propagation of the proton beam.
A linear accelerator of charged particles, notably electrons, using a travelling wave with magnetic coupling and with backward propagation is known and has been described, for example, in French document No. 2 576 477 published on 25th Jul. 1986 by the Applicant as well as in the article "Electron Linac Optimisation For Short RF And Beam Pulse Lengths" in IEEE Transactions on Nuclear Sciences, Vol-NS-32, October 1985, 3243.
Known types of linear proton accelerators, notably those referred to initially here above, comprise a plurality of types of accelerator lines such as the so-called ALVAREZ linear accelerator cavities and standing wave lines. These accelerators are limited in length, which leads to their being numerous: there are, for example, ten of them in the embodiment described here above. Often, each of these accelerators is supplied with high-frequency energy by a klystron, a fact that leads to a substantial increase in their cost.
It is another aim of the present invention, therefore, to make a travelling wave type linear proton accelerator in which a single klystron supplies high frequency energy to one or more accelerator structures arranged in series with respect to the beam. The cells of each accelerator structure should have characteristics matching the velocity attained by the protons of the beam. Thus, the accelerator structures will be of the type using travelling waves with forward or backward propagation, and their mode will be said to be fundamental or harmonic.